An electronic device is proposed in Patent Documents 1, 2, and 3, for example. A first electronic component is mounted on a first surface of a ceramic substrate. A metal plate such as a heat sink is bonded to a second surface opposite to the first surface of the ceramic substrate. A molding resin seals the ceramic substrate and the first electronic component so as to expose a surface opposite to a bonding surface of the metal plate.
Patent Document 1: JP-2003-7933-A
Patent Document 2: JP-2006-147918-A
Patent Document 3: JP-2006-222406-A corresponding to U.S. Pat. No. 7,294,912
Conventionally, as mentioned above, electronic components are mounted on one surface of the ceramic substrate. The inventors came up with the idea of mounting electronic components on the other surface opposite to a metal plate of the ceramic substrate. This makes it possible to improve the density of mounting components on a substrate. We examined prototypes based on that idea.
FIG. 30 is a schematic sectional view showing an electronic device made by the inventors as a proto type according to a related art. In FIG. 30, a metal plate 50 is arranged opposite to one surface of a ceramic substrate 10 via an adhesive J1. The ceramic substrate 10 and the metal plate 50 are bonded to each other.
A first electronic component 20 is mounted on a first surface of the ceramic substrate 10. A second electronic component 30 is mounted on a second surface. A molding resin 80 such as epoxy resin is used to seal the ceramic substrate 10, the first electronic component 20, and the metal plate 50. A bonding surface of the metal plate 50, that is, the surface opposite to the ceramic substrate 10, is exposed from the molding resin 80.
A depressed portion 50a is provided at a portion of the bonding surface on the metal plate 50 corresponding to the second electronic component 30. The second electronic component 30 is inserted into the depressed portion 50a. The second electronic component 30 in the depressed portion 50a is sealed with the adhesive J1.
The adhesive for bonding the ceramic substrate 10 to the metal plate 50 is, for example, silicon resin that is soft and flexible and shows a smaller Young's modulus than the molding resin 80. When the depressed portion 50a is not provided, the adhesive J1 becomes thicker by the height of the second electronic component 30. There may be a possibility of increasing thermal resistance and finally decreasing radiation performance.
A general method is used to manufacture such electronic device. The electronic components 20 and 30 are mounted on the ceramic substrate 10. The metal plate 50 is bonded to the ceramic substrate 10 and is placed in a die. The resin is injected into the die for sealing. This package shows a large mold structure including the ceramic substrate 10. A relatively large pressure (e.g., 7 to 20 MPa) is needed to inject the resin so as to completely fill the molding resin.
The metal plate 50 is bonded to the substrate 10 and is partially exposed from the molding resin 80. The exposed surface of the metal plate 50 is pressed against the die during the mold process. A mold pressure of the resin is applied to the ceramic substrate 10 from the one surface.
According to the above-mentioned prototype, the soft adhesive J1 is filled in the depressed portion 50a of the metal plate 50. The above-mentioned mold pressure may easily warp the ceramic substrate 10 so as to depress a portion of the ceramic substrate 10 corresponding to the depressed portion 50a toward the depressed portion 50a. The warpage of the substrate 10 may cause the substrate 10 to crack.
The above-mentioned problem may apply to not only ceramic substrates but also resin substrates such as printed substrates and metal substrates. It may be well necessary to consider a substrate crack due to the above-mentioned mold pressure applied from one surface of the substrate also in the case of half molding the substrate without bonding the metal plate to the substrate. The above-mentioned problem is considered to be common to electronic devices that half mold electronic components on both surfaces of the substrate.
Further, Patent Document 4 proposes a conventional car electronic circuit device. Specifically, Patent Document 4 proposes a device that is mounted with circuit elements on the surface side of a substrate. A lead frame island is bonded to the reverse side of the substrate. The substrate and the circuit elements are sealed with sealing resin so as to expose the island.
Such structure includes the substrate bonded with the island on the reverse side and is placed at a molding die including an upper mold and a lower mold. A resin is poured from a plunger into the molding die to mold the resin. The structure is so formed that the substrate is sandwiched between the sealing resin and the island.
Patent Document 4: JP-2001-352185 A
According to the conventional technology, however, the substrate is structured to be sandwiched between the sealing resin and the island. The substrate surface is subjected to a molding pressure during resin seal. At this time, the resin molding pressure may deform the substrate depending on a substrate state before the resin seal.
Specifically, the substrate deformation is caused by a void contained in an adhesive for bonding the island to the reverse side of the substrate. The void occurs when the adhesive is applied to the island or the reverse side of the substrate, the adhesive is hardened by hardening reaction gas, or vapor contained in the adhesive expands.
Let us consider the resin seal using the resin that contains voids as mentioned above. The lower mold presses the island. The resin molding pressure is applied to the island from the surface of the substrate. The adhesive is sandwiched between the island and the reverse side of the substrate. A strong compression force is applied to the adhesive. Accordingly, the reverse side of the substrate is subjected to a strong repulsive force.
Part of the adhesive corresponding to the void contains no adhesive and therefore causes no repulsive force of the adhesive. On the other hand, the substrate surface is subjected to a molding pressure of the resin. Unbalanced forces are applied to both sides of the substrate. An excess force is applied to the substrate and components mounted thereon. This warps the substrate, deforms the components, or causes a component failure.
Just a single void, when it is considerably big, may cause the above-mentioned phenomenon. Multiple voids, when gathered, may cause the same result.
As mentioned above, the substrate is deformed due to occurrence of a void when the substrate is so structured as to be sandwiched between the sealing resin and the island. Similarly to the void, the substrate is deformed when resin seal is applied so as to expose the reverse side of the substrate.
In this case, a gap is formed between the lower mold and the substrate. The gap functions like a void. As mentioned above, unbalanced forces are applied to both sides of the substrate to deform or damage the substrate. In particular, a ceramic substrate is easily bent at the outer edge. The bent outer edge of the ceramic substrate is pushed toward the lower mold and is destroyed due to a molding pressure of the resin.